The possibility of controlling the electromagnetic (EM)-wave transmission and reflection properties of a surface by patterning it with an appropriate array of conducting elements or apertures in a conductive grid has been well established. Dating from the late 1960s, a number of scientific and technical papers have theoretically modeled and experimentally demonstrated the performance of such structures, often referred to as Frequency Selective Surfaces (“FSS”s). FSS structures can be composed of a single or multiple layers of generally periodic conductive elements and can be designed for customized frequency response, thus the have a broad range of applications throughout the electromagnetic spectrum: they have been used as directional antennas, dichroic beam-splitters, polarizers, A/R (antireflective) and highly reflective surfaces and high- and low-pass spectral filters, etc. To date, many of these applications have involved relatively long (millimeter) wavelengths.
A principal reason for this restriction is that the feature sizes required for an FSS with controlled characteristics at any given wavelength decrease with the wavelength. Therefore, to produce an FSS for infrared (or shorter) wavelength operation, 200 nm and smaller feature sizes are needed and fabrication via electron beam lithography (“EBL”), x-ray lithography or some other ultra-high resolution means is required. These fabrication techniques are generally very slow and expensive. This has limited IR FSSs to small sizes and has severely compromised their utility for many applications (including military asset signature control).
A related device, the rectenna, a combination of rectifier and antenna, has been explored for remote power transmission via radio waves of surveillance and communications vehicles. More recently, rectennas designed for the IR-vis spectral region have been proposed for solar energy collection (conversion efficiencies of 85% have been reported for such devices operating in the RF frequencies range). As with FSS structures, the IR-vis version of this device would require periodic structures with elements having certain dimensions in the range of tens to hundreds of nanometers. For the purposes of this discussion, the term “FSS” shall also be understood to include in whole or in part antennas, rectennas and similar structures.
The conventional fabrication schemes used to produce such devices generally use the methods of semiconductor lithography to create patterns based on optical exposure of photoresist layers in a batch (discrete unit) mode. This approach has also been extended to flexible substrates under a more constrained set of conditions, but in general limited to much larger scale features. While the batch semiconductor methods are well suited for ultra-high resolution/high-density devices, they are less than optimal for use with devices requiring large flexible substrates, high throughputs and/or utilize very low cost manufacturing. Production of the precision etch masks used in semiconductor lithography typically requires a complex multi-step process which includes very uniform spin coating of a solvent-based resist, careful pre-baking, optical mask exposure, resist developing (wet or dry), rinsing and drying of the resist prior to vacuum deposition or etching, after which the mask is removed, typically by a wet stripping or etching, rinsed and dried. As features become smaller and smaller, the required shorter wavelength exposure sources and mask technology become much more complex and extremely expensive.
Nano- (and micro-) imprint (or soft) lithography (“NIL”), as generally practiced, offers a partial solution to the complexities, and therefore expense, associated with conventional batch processed photo-lithography. In this approach etch masks are produced by embossing and etching soft polymer material in batch mode. While this can require the use of expensive ultra-high-resolution lithography equipment to produce tools/stampers, these tools can be used many times, thereby allowing their high cost to be amortized over many product units. There are currently a number industrial research laboratories and academic institutions that are pursuing such techniques. However, current NIL etch pattern mask production techniques are limited in their ability to achieve low cost and relatively high throughput production volumes/sizes because they are generally being developed for the semiconductor industry, which is limited in terms of maximum substrate size and throughput.
In order to bring the advantages of semiconductor mask processing and imprint lithography to the large scale production of FSS and similar devices and structures, new techniques and methods are needed, particularly those that can be designed ultimately for compatibility with the methods of roll-to-roll manufacturing. Due to the ability to inexpensively mass-produce such structures that were hitherto prohibitively expensive in large areas through the use of continuous roll-to-roll manufacturing, this material will, for the first time, offer a cost-effective means to produce such structures for use in numerous applications: solar energy, flexible electronics, military homeland security applications (tagging, tracking and locating, camouflaging, etc.)